Have you ever tried leash-training your cat? Some take to it right away and it will be great to take breaks on your trip so he can stretch his legs safely.

Otherwise, for a car trip, I would use a larger hard plastic kennel rather than a Sherpa bag as it will give the cat more room to move around. Try to get your cat accustomed to it over the next couple of months. Feed him in the kennel so he associates it positively. Give him treats in it. Whatever you do, I don't recommend allowing the cat to roam free in the car. He could go nuts, get in the driver's way or get out from an open window or door.

As for a litter box, there are disposable ones available with the litter right in it making it very easy IF the cat takes to litter boxes easily. When I took a cat on a road trip, I used a cheap plastic box and carried a tub of litter and liners but this would have been easier. If you can, keep his regular food and water bowls so he associates them with home.

Your partner is right. IMO, do this by car rather than plane. It will be less stressful for your cat and it's not a really very long trip. I did 2 cross-countries with a dog and cat. They do fine but my cat was very vocal which was annoying but it would have been very bad on a plane. As for kitty valium, I'd leave that up to the Vet.